Organic light-emitting diodes (OLED), also known as organic electroluminescent (EL) devices, are a class of electronic devices that emit light in response to an electrical current applied to the device. The structure of an OLED device generally includes an anode, an organic EL medium, and a cathode. The term organic EL medium herein refers to organic materials or layers of organic materials disposed between the anode and the cathode in the OLED device. The organic EL medium can include low molecular weight compounds, high molecular weight polymers, oligomers of low molecular weight compounds, or biomaterials in the form of a thin film or a bulk solid. The medium can be amorphous or crystalline. Organic electroluminescent media of various structures have been described in the prior art. Dresner, in RCA Review, 30, 322 (1969), described a medium comprising a single layer of anthracene film. Tang et al., in Applied Physics Letters, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly assigned U.S. Pat. Nos. 4,769,292 and 4,885,211, report an EL medium with a multilayer structure of organic thin films, and demonstrated highly efficient OLED devices using such a medium. In some OLED device structures, the multilayer EL medium includes a hole-transport layer adjacent to the anode, an electron-transport layer adjacent to the cathode and, disposed in between these two layers, a light-emitting layer. Furthermore, in some preferred device structures, the light-emitting layer is constructed of a doped organic film comprising an organic material as the host and a small concentration of a fluorescent compound as the dopant. Improvements in EL efficiency, chromaticity, and lifetime have been obtained in these doped OLED devices by selecting an appropriate dopant-host composition. The dopant, being the dominant emissive center, is selected to produce the desirable EL colors. Examples of the doped light-emitting layer reported by Tang et al. in commonly assigned U.S. Pat. No. 4,769,292 and by Chen et al. in commonly assigned U.S. Pat. No. 5,908,581 are tris(8-quinolinol)aluminum (AlQ) host doped with coumarin dyes for green emitting OLEDs, and AlQ doped with 4-dicyanomethylene-4H-pyrans (DCMs) for orange-red emitting OLEDs. Shi et al., in commonly assigned U.S. Pat. No. 5,593,788, disclose that improved EL efficiency and color was obtained in an OLED device by using a quinacridone compound as the dopant in an AlQ host. Bryan et al., in commonly assigned U.S. Pat. No. 5,141,671, disclose a light-emitting layer containing perylene or a perylene derivative as a dopant in a blue emitting host. They showed that a blue emitting OLED device with an improved EL efficiency was obtained. In both disclosures, the incorporation of selected fluorescent dopants in the light-emitting layer is found to improve substantially the overall OLED device performance parameters. Co-doping of light-emitting layer with anthracene derivatives results in devices with better EL efficiency as shown in JP 11-273861 and JP 07-284050.
The most common formulation of the doped light-emitting layer includes only a single dopant in a host matrix. However, in a few instances, incorporation of more than one dopant in the light-emitting layer was found to be beneficial in improving EL efficiency. Using a light-emitting layer containing rubrene, a yellow emitting dopant, and DCJ, 4-(dicyanomethylene)-2-methyl-6-[2-(4-julolidyl)ethenyl]-4H-pyran, and a red emitting dopant in an AlQ host, it is possible to produce a red emitting OLED device with improved EL efficiency and color; see Hamada et al. in Applied Phys. Lett. 75, 1682 (1999), and EP 1 162 674 B1. Here rubrene functions as a co-dopant in mediating energy transfer from the AlQ host to the DCJ emitter.
Doping light-emitting layers with materials that are primarily hole-transporting compounds having the function of assisting in transportation of the charge carriers (holes) to improve luminous efficiency has been described, for example, by Mori et al. in commonly assigned U.S. Pat. No. 5,281,489, by Aziz et al. in commonly assigned U.S. Pat. No. 6,392,339, by Hatwar et al. in commonly assigned U.S. Pat. No. 6,475,648, and by Matsuo et al. in EP 1 231 252 A2. These references disclose that high concentrations of hole-trapping materials, for example 50% or more, are required to provide the reported operational improvements. It has been disclosed by Hamada et al. in U.S. patent application Publication 2004/0066139 A1 that the hole-transporting materials present in a light-emitting layer at concentrations less than 5% cannot satisfactorily function as an auxiliary dopant.
Luminous efficiency of OLED devices remains a factor limiting possible OLED applications and competitiveness. Developing advanced materials and device configurations play an important role. It has been observed that use of different materials in OLEDs, most critically hosts and dopants in light-emitting layers, leads to different device performance parameters, the most basic of which are drive voltage, CIE coordinates, electroluminescence (EL) efficiency, and operational lifetime.
Although EL efficiency and color have been improved significantly using doped light-emitting layers of various compositions, the problem of insufficient EL efficiency persists. Insufficient luminous efficiency presents an obstacle for many desirable practical applications.